Reaction Mechanism. Part 5 of 8

Hyperconjugation:Baker and Nathe in 1935, while investigating the mechanism of thereaction between p-substituted benzyl bromide ad pyridine observed thatthe reaction rate increases as the electro negativity

ofthe group decreases. In other word if the group X is capable ofwithdrawing electrons from the benzene ring, reaction slows up; whilethe opposite group speed it up. Now since the inductive effect of thevarious alkyl group followings the following order,

                                         Me₃C- > Me₂CH- > CH₃.CH₂- > CH₃-

Thereaction rate should also increases in the same order when X issubstituted by these alkyl groups. But actually the two workers foundexactly the opposite reaction rate which coincided with

                                        -CH₃ > CH₃.CH₂- > (CH₃)₂CH- > (CH₃)₃C-

theabove order. It thus appears that the alkyl groups are capable ofgiving rise to electron release by a mechanism, different from theinductive effect, in which methyl group is the most successful exponentwhile the tert-butyl group is the least.

Theconcept of hyperconjugation involves the conjugation between thes-electrons of a single carbon hydrogen bond and p-electrons of theadjacent multiple bond. It is therefore, also called s-p conjugation. This type of electron release due to the presence of the system H-C-C=C is know as hyperconjugation.

-Morethe number of H-C bonds attached to the unsaturated system more will bethe probability of electron release by this mechanism. Thus theelectron release by this mechanism will be greater in methyl(possessing three Hyperconjugated H-C bonds), less in ethyl (containingtwo such bonds) and isopropyl (one) and essentially zero intertiary-butyl (no hyperconjugated H-C bond) group.

Note-Itis important to note that although Hyperconjugation like the inductiveeffect cause the release of electrons and thus the two effectsreinforce each other in this respect, the magnitude of the two effectschanges in opposite direction in the passing along a series of alkylgroups.

Theoretical Interpretation of Hyperconjugation: The phenomenon of Hyperconjugation may be considered in terms of resonance as well as molecular orbital theory.

(i) Resonance:Hyperconjugation may be interpreted in terms of resonance. For example,propylene may be regarded as the resonance hybrid between the followingstructures

Thehydrogen atom carries the positive charge but there is no bond betweenthe hydrogen and the carbon atom. For this reason, resonance of thistype is called no-bond resonance.

(ii) In terms of Molecular Order Concept:Hyperconjugation is the delocalization of the electron brought aboutthe side way overlapping of p-orbitals of the double bond with sorbital of the a–C –H bond of the alkyl group. While considering themolecular orbital treatment of the process of Hyperconjugation, it spresumed the s-orbital to the sp³ hybrid orbital overlaps tosome extent with the p-orbitals on the adjacent carbon atoms to formsomething like a non-localised  

Inother words, Hyperconjugation arise from the overlap of the C-H sigmabonding electrons of the alkyl group with p-electron of the double bond.

Effects of Hyperconjugattion: Hyperconjugation can affect both physical and chemical properties of molecules which is evident from the following examples.

(i) Heat off Hydrogenation:It is well known fact for the compounds of similar structure, thesmaller the heat of hydrogenation or heat of combustion is, the morestable is the compound.

Onthe same basis, I explained the more stability of isobutene (heat ofcombustion=646.1 kcal/mole) than the isomeric 1-butene (heat ofcombustion=649.8 kcal/mole)

Thuson the basis of hyperconjugation, it is explained greater the stabilityof the olefins in which the double bond is certainly situated ascompared with the isomeric olefin having terminal double bond Forexample, 2-pentene is more stable than 1-pentene by 2.3 kcal/mole

Similarlyamong the two isomeric forms of 2- methyl butanes, viz.2-methylbutene-2 and 2-methyl-butene-1, the 2-methyl-butene-2 havingcentrally situated double bond is stable than the latter havingterminal double bond since in the former there are nine a-hydrogenatoms which can undergo hyperconjugation as compared to only five inanother.

(ii) Bond lengths:Hyperconjugation also affects bond lengths because during the processthe single bond in compound acquires some double bonded character.Hyperconjugation is suggested as a key factor in shortening of sigmabond (? bonds) in such systems.

Forexample, The shorten distance between 1,3-butadiene and methylacetylene approximately 1.46 angstrom in length, much less than thevalue (1.54 angstrom) found in saturated hydrocarbons. This is duemainly to hyperconjugation that gives partial double-bond character ofthe bond.

(iii) Dipole Moments: Since Hyperconjugation causes the development of charges; it also affects the dipole moment of the molecule

-Thefact the saturated aldehyde have nearly same dipole moment indicatesthat the phenomenon of hyperconjugation is also responsible for thedipole moment.

Incase aldehyde where the effect of Hyperconjugation is decreased (due todecrease in the number of a-C-H bonds) the effect of induction isdecreased with the result the dipole moments.

(iv) Ionization Potential:Since hyperconjugation increases the electron density on olefiniccarbon atoms, it is not beyond expectation that ionization potential ofolefins decreases with the increase in hyperconjugation. For example- H₂C=CH₂ (10.62 volts), CH₃CH=CH₃ (9.76 volts), CH₃-CH=CH-CH₃ (9.34), (CH₃)₂-C=CH₂ (9.26 volts) respectively.

Hyperconjugationalso accounts for the drop ionization potential of the C=O group offormaldehyde as its hydrogen atoms are replaced by alkyl groups.

(v) Carbonium Ion Stability:  The well established decreasing order of stability of carbonium ions, i.e.,

Tertiary>secondary> primary

iscommonly attributed to hyperconjugation. In general greater the numberof hydrogen atoms attached to a-carbon atoms, the more hyperconjugativeform can be written and thus greater will be stability of the carboniumion.

Free radicals also have a similar order of stability:

t-Butylfree radical is the most stable because it can have ninehyperconjugative forms, while isopropyl free radical have six, ethylradical has only three and methyl nil hyperconjugative forms.

Similarly,Hyperconjugation explains the decreasing order of stability of primarycarbonium ions with increase in number of methyl groups on the a-carbonatom.

(vi) Orienting Influence of Methyl Group:The o, directing influence of the methyl group in methyl benzene isattributed partly to inductive and partly to hyperconjugative effect.

-The o- and p-directing influence of methyl or any other alkyl.

Itis clear from the above case the electron density at o- and p-positionw.r.t the methyl group increases due to hyperconjugation and thereforeelectrophilic substitution in toluene will occur at o and p-positionw.r.t methyl group.

(vii) Addition of Hydrogen Bromide of Alkyl bromide: Alkyl bromide when treated with hydrogen bromide forms a mixture of 1,2 and 1,3-dibromides.

Formationof 1,3-dibromide is in accordance with the inductive effect exerted bybromine atom. Due to –I effect of bromine the compound also undergoeselectromeric effect towards the bromine atom and thus HBr adds in thefollowing way to form 1,3-dibromide.

Formationof 1,2 dibromide can only be explained on the basis of hyperconjugationowing to which the compound undergoes electromeric effect away from thebromine atom and thus HBr adds in the following way to form1,2-dibromide

Howeversince the –I effect of bromine atom is very weak than thehyperconjugative effect, the main product is that which is governed byhyperconjugation (1,2-dibromide).